1. Field of the Invention
The present invention relates to a zoom lens system which is mainly used in an electronic still camera (digital camera), and has a wide-angle range (a half angle-of-view of more than 30°), such as a zoom ratio of about 2 or 3.
2. Description of the Prior Art
In recent years, there has been an increasing need for further miniaturization and higher definition in digital cameras; and the pixels of CCD imaging devices are being further miniaturized. Accordingly, photographing lens systems of such digital cameras are required to have high resolution; and a long back focal distance is also required to accommodate a filter group. Furthermore, an optical system for a color CCD is required to have good ‘telecentricity’ in order to prevent shading and color shift, i.e., the light rays which exit from the final lens surface and are incident on the imaging surface are to be as perpendicular to the imaging surface as possible.
As a miniaturized zoom lens system for a compact digital camera, a front-negative lens system (negative-lead type lens system, i.e., telephoto type lens system) can be used in the case where a zoom ratio is up to around 2 to 3. Such lens systems can provide a wider angle of view at the short focal length extremity, and can achieve further miniaturization of the lens system. In particular, the lens diameter of the first lens group (the most object-side lens element) can be made smaller. Due to the above features, the negative-lead type lens system is suitable for a ‘telescoping’ zoom lens system in which the distance between the lens groups is made shorter in an accommodating position. Furthermore, since the exit pupil position needs to be sufficiently distant from the image plane, a zoom lens system of three-lens-group arrangement, including a negative lens group, a positive lens group, and a positive lens group, in this order from the object, is often utilized. Such three-lens-group zoom lens systems have been disclosed in, e.g., Japanese Unexamined Patent Publication Nos. Hei-10-213745 and Hei-10-170826. Applicant notes that Hei- 10-170826 is a family member of U.S. Pat. No. 6,124,984, which issued to Shibayama on Sep. 26, 2000.
Japanese Unexamined Patent Publication No. Hei-10-213745 has achieved miniaturization to a certain extent by reducing the number of lens elements; however, the front lens diameter and the overall length of the optical system with respect to the focal length is still large, so that adequate miniaturization is not achieved. Moreover, U.S. Pat. No. 6,124,984 teaches a telecentric optical system which achieves miniaturization; however, the optical system includes seven lens elements, which is relatively a large number of lens elements, so that the length of the optical system in the accommodating position becomes long, which increases the size of a camera. In a zoom lens system for a small camera having a telescoping lens barrel, the thickness of each lens group is required to be smaller, in addition to the front lens diameter and the overall length of the lens system which are required to be miniaturized in order to reduce the size of the camera body. Generally, if attempts are made to reduce the number of lens elements in order to miniaturize the lens system, and to reduce the thickness of the lens groups, the correcting of aberrations becomes more difficult. Accordingly, in order to adequately correct aberrations over the entire zooming range while achieving miniaturization, an appropriate refractive power distribution for each lens group and an appropriate lens arrangement are necessary.